Illumination devices based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature one or more lighting units, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects. These lighting units may employ two or more groups or “channels” of LEDs which produce light of different colors, each supplied with the proper current to enable generation and mixing of light to produce a desired lighting effect, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference.
In some lighting units, the first channel may include a first plurality of white LEDs (e.g., four LEDs) in series with each other, and the second channel may include a second plurality of red LEDs (e.g., two LEDs) in series with each other. A desired color effect of the lighting unit may be controlled by adjusting the current through the two channels. In some lighting units the channels are connected in series so that a single stream or channel of current flows through all the LEDs, and a shunt is provided across selected LEDs (e.g., the LEDs of the second channel) to divert the current away from selected LEDs to yield the desired color effect.
Unfortunately, this arrangement typically entails a loss of energy and/or a complicated control scheme. For example, if the shunt is a linear shunt, it can result in additional unwanted power losses. A switching or pulse-width modulated (PWM) shunt can be employed, but the known arrangements require a complicated drive scheme.
Thus, there is a need in the art to provide a lighting unit with multiple LED channels which can be driven efficiently to achieve a desired lighting effect.